The motive power generated by the power equipment (engine or motor) of the automotive vehicle is transmitted to wheels via a power transmission system. The power driving mode of vehicles comprises three modes of front-wheel drive, rear-wheel drive and front-rear-wheel drive. The drive shaft is an indispensable component in the power transmission system no matter which drive mode is used.
In order to have enough driving force when vehicles drive on road surfaces with low adhesion coefficients, usually power transmission systems of vehicles are designed to be front wheel and rear wheel drive mode (namely front-rear-wheel drive mode). Especially trucks usually drive on road surfaces with bad road conditions, so the front-rear wheel drive mode is used mostly.
The power transmission path of front-rear-wheel drive vehicles is the power is transmitted to the transfer case, then the transfer case distributes the power into two part to output: one part is for the front drive shaft to drive the front axle, the front axle drives the front wheels via the front axle shaft; the other part is for the rear drive shaft to drive the rear axle, the rear axle drives the rear wheels via the rear axle shaft. For light-duty vehicles without independent suspension systems, the front axles are not designed, the front drive shafts drives the front wheels directly. For front-wheel drive or rear-wheel drive vehicles, the power is not transmitted to the drive shafts of wheels which do not participate in driving.
The transfer case is not designed in some vehicles, however a combinatory transmission is used to replace the transfer case to distribute the power. In China, the patent Front and Back Driving Force Output Structure of Automobile Tractor Without Independent Transfer Case with publication number of 101659211A and publication date of 3 Mar. 2010, discloses a driving force output structure of front-wheel and rear-wheel drive vehicles which replaces the transfer case with a combinatory transmission.
The drive shaft (a general term for front drive shaft, front axle shaft, rear drive shaft and rear axle shaft and so on) is designed to have corresponding load-carrying capability for vehicles with any power output structure, during the use process, if the load on the front wheel or rear wheel is greater than the set point, the corresponding drive shaft will be fractured to prevent overload which damages the front and rear axles, the transfer case, the engine and so on (as in the whole power output system, the cost of the drive shaft is the lowest and is convenient to be replaced, when one component has to be damaged, the drive shaft is the first choice). However, particularly in all-wheel-drive vehicles, when vehicles turn or drive on road surfaces having concave pits and the front wheels fall into the concave pits, the front wheels will have instantaneous impact, namely instantaneous overload which will cause fracture of the front drive shaft or the front axle shaft. Such instantaneous overload can not be avoided by the driver by controlling the load capacity the fracture of the front drive shall or the front axle shaft caused by instantaneous overload is also not needed by the designer or user, but a design disadvantage existing for a long time in the vehicle design; moreover, when the vehicle is overloaded, the design mode that a component has to be damaged (at present the damaged component is the drive shaft) is a technology bias existing for a long time in the power transmission system design of the vehicle, if the drive shaft is damaged in the open air, the expanse to send the vehicle to the repair factory or the labor cost and traveling expanse to ask maintenance men to repair is far more than the material cost of the drive shaft, therefore the design concept increases the use cost and bring great inconveniences to users; as the prior drive shaft is an integrated stiff shaft, when the resistance of the vehicle is changed, the resistance is reacted on the engine directly and causes unsteady of the engine load and poor stability of the engine, thus the fuel consumption is also increased, the principle is the same as when a driver which can not drive the vehicle steadily, the fuel consumption is high.